JP3939652B2 - Multidimensional ultrasonic transducer array - Google Patents

Description

[0001]
The present invention relates to transducer probes for ultrasound diagnostic imaging systems, and more particularly to probes having elements extending in two or more dimensions.
[0002]
An element in the acoustic element array used for ultrasonic imaging is excited by applying a potential to both ends of the element with electrodes connected to the opposite surface of the element. Due to the applied potential, the piezoelectric element vibrates and emits ultrasonic waves. During reception, the element is vibrated by sound waves that are converted into electrical signals, which are sent to the imaging system by the same electrodes used to excite the elements. Piezoelectric elements are one of a single array of elements in a transducer array such as those used for two-dimensional ultrasound imaging, and there are many options for connecting the electrodes to the opposite surface of the elements. In particular, it is common to make connections from the sides of the array, separated from acoustic backing blocks that attenuate unwanted acoustic energy on the back side of the array.
[0003]
However, when the transducer array constitutes a two-dimensional array, it becomes difficult to make all connections from the sides of the array to the elements. This is because in the case of an array of three or more columns of elements, one or more columns are located inside the array and access is blocked by columns outside the array. In such cases, it is generally necessary to make a connection through an acoustic backing of the transducer stack.
[0004]
One prior art approach to making a connection to a two-dimensional array is disclosed in US Pat. No. 4,825,115. In this patent, a flexible printed circuit (flex circuit) is attached to the back side of a piezoelectric element plate and is bent upward so as to extend perpendicular to the plate from the attachment position. The acoustic backing is cast around the flex circuit, the assembly is turned over, and the piezoelectric plate is diced into individual elements. US Pat. No. 5,757,727 discloses a similar approach, but attaches the flex circuit to the complete array area, casts the backing into a unitary assembly, and the individual sub-assemblies of the backing and flex circuit are preformed. And then assembled together to complete a row of assemblies. This approach requires tightly coordinated uniformity from row to row to ensure constant performance and conductor spacing. Furthermore, both of these techniques require the conductor to bend 90 degrees at the mounting position on the transducer element, which causes stress on the conductor, leading to impedance variations and connection failures.
[0005]
U.S. Pat. Nos. 6,043,590 and 6,444,533 disclose an approach that avoids the need to bend the conductors. Instead, the conductors are vertically bent against the back side of the element and are not bent. In addition, the conductor and backing material can be preformed as a unitary assembly and inspected for necessary inter-element alignment prior to attachment to the piezoelectric blocking material. In the latter case, dielectric substrates having exposed conductor window portions are laminated, and the space between the conductors is filled with a damping material. When the material is cured, the fixed laminate is cut through the material to create a backing block with conductors that terminate at the cut surface. However, this process tends to be difficult to maintain the alignment of the bare conductors and to completely fill the surrounding space without leaving voids. Therefore, it would be desirable to be able to make a conductive backing assembly as described above in a simple, accurate and highly repeatable manner.
[0006]
In accordance with the principles of the present invention, an ultrasonic transducer probe includes a two-dimensional array of acoustic elements, with conductors attached to the acoustic elements by a conductive backing block assembly. The assembly includes a plurality of alternating flex circuits and plates of backing material that are adhesively bonded to form a unitary assembly. Alternatively, conductive traces can be formed directly on each plate bonded together. The assembly can be easily and accurately manufactured using conventional transducer assembly processes without bending the conductors.
[0007]
Referring to FIGS. 1a-1c, steps for assembling a conductive backing block for a two-dimensional (2D) ultrasonic array transducer stack are shown from a perspective view. The backing block assembly has two main components: a flex circuit 12 and a backing block plate 14. A flex circuit includes a separate substrate, such as a Kapton sheet, on which a plurality of conductive traces (wiring patterns) are formed, typically by fot etching. Typically, the Kapton sheet has a thickness of 1-3 mils (25 μm-75 μm). In FIG. 1a, the trace 16 is shown ending at the top of the Kapton substrate, but in the constructed embodiment, as shown in the micrograph discussed below, slightly beyond the upper edge of the substrate. It may be desirable to have traces that extend. The trace extension offsets the substrate from the contact between the trace and the transducer element, reducing defects due to thermal expansion of the Kapton at the junction. The acoustic impedance behind the transducer element is well controlled and Kapton particles are reduced or completely removed from grinding and dicing. The lower termination of trace 16 typically ends with a conductive electrode terminal (not shown), allowing the trace to connect to other printed circuits and components.
[0008]
The backing block plate 14 is formed from a sheet of backing material having a predetermined thickness. The backing block is typically cast from an epoxy mixed with ultrasonic absorbers and scatterers such as microballoons and small particles. The mixture of these materials is adjusted to impart a predetermined acoustic impedance and attenuation to the backing block, as is known. The backing block plate 14 has a predetermined thickness obtained by a controlled grinding process.
[0009]
In accordance with the principles of the present invention, the conductive backing block assembly is composed of alternating layers of flex circuits 12a-12c and backing material plates 14a-14d, as shown in the exploded view of FIG. 1b. Are laminated together by an adhesive such as Plates 14a-14d are preferably cut from the same sheet of backing material and ground to the same adjusted thickness so that all plates have the same components and therefore exhibit the same acoustic properties. In the constructed embodiment, the assembly is cured under pressure in a hot press. Squeezing squeezes out excess epoxy, causing alternating flex circuits to be evenly spaced in the elevation dimension and expelling bubbles from the layers. When the assembly is cured, the top surface to which the connection to the transducer element 20 is made is ground to a smooth finish, preferably gold plated for attachment to the underside of the transducer element, and the transducer element is also preferably gold plated. A preferred method for connecting the conductive backing block assembly to the transducer elements is by adhesive attachment using a low viscosity adhesive such as epoxy. The low viscosity creates a direct ohmic contact between the gold plating of the conductive backing block assembly and the transducer element while at the same time forming a secure adhesive bond between the two surfaces. The assembled transducer stack 10 is shown in FIG. 1c, with arrows on the right indicating the azimuth (Az) and elevation (El) dimensions. The row of traces for each flex circuit 12 typically constitutes the azimuth dimension of the array, and the spacing between flex circuits constitutes the elevation dimension, but vice versa. A common use is generally the case for 1.5D arrays that are oriented and focused on a single azimuth plane but only focused on the elevation plane. Orientation and focusing are realized in the front volume of the array. 2D arrays for 3D imaging do not have definable azimuth and elevation dimensions and are therefore generally expressed in polar coordinates. It is. However, this term is used in this application for clarity and to maintain consistency of references to the drawings.
[0010]
It will be appreciated that the assembly of the present invention may alternatively be constructed using a rigid (eg, FR4) printed circuit instead of a flexible printed circuit. Flex circuits are preferred because of their thin shape and ease of manufacturing to form traces that extend beyond the substrate.
[0011]
2a-2c show a second embodiment of the present invention. In this embodiment, there is no flex circuit. Instead, the conductive traces 16 are formed directly on the backing block plate 14, which may be done by a photoetching process. Therefore, the spacing (pitch) in the elevation direction of the transducer elements will not include the thickness of the flex circuit board of this embodiment. As mentioned above, the trace 16 may terminate with electrode terminals interconnecting at the bottom, but it may not need to extend beyond the top edge of the plate 4. The conductive backing block assembly is formed as shown in the exploded view of FIG. 2b. In this particular embodiment, the backing block plates 14a-14h have positively different lengths and are connected to all ends of the traces (in this example, not extending beyond the bottom of the plate). It may be easy. Alternatively, the backing block plate has the same length so that the trace ends at the lower surface of the backing block assembly in the same manner as it ends at the upper surface. In such a case, the trace may terminate at an electrode terminal grid array on the lower surface for connection by the connector, and the connector connects to the lower surface of the assembly and its trace. The two central plates 14a, 14b in the illustrated example are half as thick as the plates surrounding them, and the traces on the opposite side of the plates are the same number of transducer elements. To be centered with respect to the two middle columns. No elements are formed on the end cap plates 14g, 14h and they are used only to surround the rest of the assembly while supporting the outermost element rows. Final processing of the conductive backing assembly and attachment of the transducer is performed as described above. The completed transducer stack 30 is shown in FIG. 2c as a partial assembly in a partially isolated view to reveal trace alignments that traverse within the top surface of the conductive backing assembly.
[0012]
Figures 3a-3c show in detail the configuration of the transducer stack of the present invention with a conductive backing block assembly. In these drawings, conductive backing block assembly 50 includes alternating layers of flex circuit 12 and backing plate 14, which is formed from a backing plate with conductive traces as described above. sell. In FIG. 3a, the conductive backing block assembly is gold-plated on the top and attached to the piezoelectric element plate 22 which is gold-plated on the top and bottom. In this assembly 50 below the piezoelectric element plate, there are three flex circuits 12a, 12b, 12c extending, for example, in the azimuth dimension. In FIG. 3b, the piezoelectric element plate 22 is diced by two cutting parts 24a and 24b in an azimuth dimension, so that three rows of piezoelectric elements 22a respectively located above the respective flex circuits 12a, 12b and 12c. , 22b, 22c are formed. The cut extends through the gold-plated connection between the assembly 50 and the piezoelectric element 22 and thereby electrically isolates the electrical connection of the flex circuit below each row of piezoelectric elements. .
[0013]
The two matching layers 26a and 26b are located on the piezoelectric element as shown in FIG. 3c. The matching layer adapts the acoustic impedance of the transducer to the body that transmits and receives the acoustic signal. Prior to laminating the matching layer, a conductive sheet (not shown) may be disposed on the top surface of the piezoelectric element that is gold-plated as described above. Preferably, the surface of the matching layer 26a that connects to the piezoelectric element is metallized for connection to the top surface of the transducer element. In FIG. 3d, the array of piezoelectric elements is diced in elevation dimension completely through the gold-plated connection between assembly 50 and piezoelectric element 22, as indicated by reference numeral 30, to form individual transducer elements. At the same time, the gold-plated contact portion on the lower side of each transducer element is electrically separated. As indicated by reference numeral 28, these cuts extend without completely penetrating the lower matching layer 26a, thereby providing a continuous sheet of conductive sheet across each line of the element in the elevation dimension. Strips are left behind. Thus, electrical connections to the upper electrodes of all elements, including elements inside the array, can be performed from the elevation side of the array.
[0014]
In this particular example, a sub-diced element is formed so that each pair of sub-diced elements adjacent in the azimuth direction operates as an integral element for better high frequency performance. . One such pair consists of sub-elements 20a, 20b, and a single trace of the underlying flex circuit 12a as indicated by the projection of the Y-shaped conductor 36 of flex circuit 12a on the side of assembly 50. Connect to. The Y-shape at the top of the lead that distributes the lead to each sub-element allows the cut 30 to be made in the assembly 50 without the dicing saw being caught by the flex circuit lead bit. In addition to being sub-diced in the azimuth direction, sub-dicing may be performed in the element elevation direction to improve acoustic performance.
[0015]
FIGS. 4a-4e illustrate a transducer stack of the present invention that will operate in the k ₃₁ mode, for example as disclosed in US patent [Application Serial No. 09 / 457,196 filed Nov. 3, 1999]. The configuration is shown. Rather than vertical conventional excitation between the bottom top of the device (the side that faces the patient), in k ₃₁ mode, the transducer elements are polarized and excited in the transverse direction. This allows the device electrodes to be located on the side of the device rather than on the top and bottom. In the example of FIG. 4a, the piezoelectric element plate 22 is adhesively bonded to a conductive backing block assembly 50 including embedded flex circuits 12a, 12b, 12c, but with a conductive wire etched as described above. Can be included. Unlike the example of FIG. 3, in this embodiment, there is no gold-plated electrode between the piezoelectric element plate and the assembly 50, and the piezoelectric element is simply attached to the finished surface of the assembly 50. In FIG. 4b, the piezoelectric element plate 22 is diced in an elevation dimension to form rows of piezoelectric element material that traverse the columns of the backing block and flex circuits 12a, 12b, 12c. These dicing cuts 30 are made with conductive traces on the underlying flex circuit so that the ends of the traces are located at the bottom of the cuts 30. In FIG. 4c, the laterally opposing walls 32 in the cutting section 30 are plated with electrode material, which may be coated by wet plating, vapor deposition or sputtering. This electrode is attached not only to both lateral piezoelectric element walls 32 of the dicing cut 30 but also to the bottom of the cut where the conductive trace ends. Thus, this electrode electrically connects the conductive traces at the bottom of the cut to the lateral sides of the piezoelectric element on both sides of each cut.
[0016]
In FIG. 4d, the matching layers 26a, 26b are covered. In this embodiment, all connections are made from the bottom through the flex circuit leads, so no plated electrodes or sheets are required on top of the piezoelectric elements. The 2D array, as shown in FIG. 4e, 42a, to dice the matching layer in the elevation dimension to the previous cut 30 and to form different rows of individual transducer elements extending in the azimuth dimension. Completed by dicing the rows of piezoelectric elements in the azimuth dimension as shown in 42b. The dicing cut portions 42a and 42b are formed up to the inside of the upper surface of the conductive backing block 50, and pass through the conductive material at the bottom of the cut portion 30 so as to electrically isolate each row of elements. To. Sub diced pair of sub-elements are operated in k ₃₁ mode by a connection from flex circuit conductors of the cutting unit for continuous column, through the traces of the flex circuit in the downward signal (hot) and return ( Provide alternate (ground) paths. For example, the transducer elements formed by the sub-elements 20a, 20b are laterally opposed to connect to the conductors 38 on the flex circuit 12a below the column of the elements as shown in the projection in FIG. 4e. It has an electrode surface. The opposite side surface of the sub-element is connected to the conductive wire 34 on the flex circuit 12a, and a dicing cut portion 30 ′ (not shown) that supplies a common potential or a ground potential on the other electrode side surface of the sub-element. ) Connect to the conductor terminated at the bottom. Thus, all electrical connections to the transducer elements can be realized through the conductive traces of the conductive backing block 50.
[0017]
If the transducer element connects to the plated surface of the conductive backing block at the bottom, a complete conductor alignment is not required, and the combination of conductive traces and plating on the surface allows a transducer stack from the manufacturing process. High productivity. For example, FIG. 5a is a plan view of the gold-plated surface 60 of the backing block, intersected by the ends of the conductive traces 16a, 16b, 16c that penetrate the conductive backing block. Four horizontal rows of conductive traces extending from four horizontally arranged flex circuits or backing plate surfaces are shown. From this it can be seen that the top, second and bottom rows are in a vertical alignment in this example, but the third row including the conductive trace 16b is not in a vertical alignment with the other rows. When the piezoelectric element plate is attached to the plated surface 60 and centered with respect to the aligned conductive traces and diced into separate transducer elements, the plated surface is at the bottom of the element as shown in FIG. 5b. It is divided into plating areas that fit the seating area (footprint). The plating area is divided by the dicing cut portions 30 and 40. It can be seen that the conductive traces in rows 12a and 12c are well aligned to the center of the plating area of the seating area of each element, as intended. The non-aligned conductive trace rows 16b are not aligned at the center of the plating area, but each will still intersect the intended plating area and will still perform the desired function. Even traces such as 16d that are dramatically off-center will still provide a satisfactory electrical connection to the plated area. In a particular embodiment, the plated area may have a thickness of about 0.5 μm, the outer dimensions are on the order of 200 μm × 200 μm, and the width of the conductive trace 16 is on the order of 50 μm, Often, a 4: 1 placement tolerance is given to the trace in each orthogonal direction. The accuracy in the elevation direction is maintained by adjusting the thickness of the backing block plate as it is ground to the desired thickness. A sub-diced element may have two sub-elements with dimensions of 125 μm × 250 μm, still allowing a relatively wide tolerance. As the transducer element and plating area seating areas decrease and approach 50 μm × 50 μm, the conductive traces are expected to be correspondingly smaller.
[0018]
FIG. 6 is a microphotograph before plating of the top surface of the conductive backing block of the present invention. The micrograph clearly shows alternating horizontal rows of flex circuit 12 and backing block plate 14. The end of the conductive trace 16 extending from the flex circuit is clearly visible in the micrograph. The black areas between these conductive traces 16 in each row are vacancies filled with an epoxy adhesive that bonds the assembly together. In this view, the end of the conductive trace extends on the Kapton substrate to its end at the surface of the conductive backing assembly.
[0019]
The microphotograph also shows that the flex circuit columns are alternately arranged in a staggered pattern from column to column. This is because this particular conductive backing assembly is designed for hexagonal 2D array transducers that repeat a triangular relationship with each other such that the transducer elements form a hexagonal collection. Such a 2D hexagonal array is disclosed, for example, in US Patent [Application Serial No. 09 / 488,583, filed Jan. 21, 2000]. Thus, the present invention is applicable to linear 2D arrays with configurations such as hexagonal arrays and other shapes.
[0020]
Although the illustrated embodiment is shown using a piezoelectric transducer, the present invention is capacitive and piezoelectric microfabricated that may be electrically connected via a conductive backing block assembly. Other transducer technologies such as micromachined transducers (Cmuts and Pmuts) are equally applicable. A Cmut type transducer is disclosed, for example, in US Pat. No. 5,619,476.
[Brief description of the drawings]
1a-1c illustrate a first embodiment of an ultrasonic transducer stack with a conductive backing block constructed in accordance with the principles of the present invention. FIG.
FIGS. 2a-2c illustrate a second embodiment of an ultrasonic transducer stack with a conductive backing block constructed in accordance with the principles of the present invention. FIG.
FIGS. 3a-3d illustrate an ultrasonic transducer stack with chopped transducer elements constructed in accordance with the principles of the present invention. FIGS.
[4] Figure 4a-4e illustrate an ultrasonic transducer stack with a transducer element which operates on the principle is constituted by k ₃₁ mode of the present invention.
Figures 5a and 5b show the alignment of the conductors in the backing block of the present invention relative to the seating area of the transducer element.
FIG. 6 is a micrograph of the conductive backing block of the present invention for a two-dimensional hexagonal array.

Claims (14)

A two-dimensional array of ultrasonic transducer elements having a bottom surface that emits undesirable ultrasonic energy;
Mounted to face the bottom surface of the two-dimensional array, viewed contains a separation plate and a set of circuit conductive traces formed by alternately bonded conductive backing block assembly of acoustic backing material,
Conductive traces of the circuit terminate at the surface of the conductive backing block assembly opposite the two-dimensional array;
The surface of the conductive backing block assembly where the conductive trace terminates is conductively plated, the conductive plating electrically connecting to the conductive trace;
The two-dimensional ultrasonic transducer probe, wherein the conductive plated surface is electrically divided into separate areas corresponding to the seating areas of the elements of the array transducer when the transducer array is cut .   The set of conductive trace circuits is a printed circuit board with conductive traces, and the spacing plate and the printed circuit board are located between a coupling surface of the plate and the printed circuit board. The two-dimensional ultrasonic transducer probe of claim 1, which is bonded together by an adhesive.   The two-dimensional ultrasonic transducer probe according to claim 2, wherein the printed circuit board constitutes a flex circuit.   4. The two-dimensional ultrasonic transducer probe of claim 3, wherein the flex circuit extends beyond the end of the plate at one end of the conductive backing block assembly that does not face the two-dimensional array.   The two-dimensional ultrasonic transducer probe of claim 1, wherein the set of conductive trace circuits are formed on the spacing plate that are adhesively bonded together.   The two-dimensional ultrasonic transducer probe of claim 5, wherein the plates have different lengths to provide access to the conductive traces. The conductive traces terminate in a grid array of electrode terminals on the surface of said conductive backing block, in that grid array connection to the assembly is made, according to any one of claims 1 to 6 Two-dimensional ultrasonic transducer probe. 8. The two-dimensional ultrasonic transducer probe according to any one of claims 1 to 7 , wherein the two-dimensional array is a two-dimensional array of micromachined ultrasonic transducer elements having a bottom surface that emits undesirable ultrasonic energy. The two-dimensional ultrasonic transducer probe of claim 8 , wherein the micromachined ultrasonic transducer element comprises a capacitive micromachined ultrasonic transducer element. The two-dimensional ultrasonic transducer probe according to claim 9 , wherein the micromachined ultrasonic transducer element includes a piezoelectric micromachined ultrasonic transducer element. Plate made of the acoustic backing material, having a selected thickness so as to establish a predetermined distance in the elevation direction between the circuit traces of the conductive, according to any of claims 1 to 10 Two-dimensional ultrasonic transducer probe.   The two-dimensional ultrasonic transducer probe according to any one of claims 1 to 16, wherein the plate of the acoustic backing material contains an acoustic absorbing material and an acoustic scatterer. The two-dimensional ultrasonic transducer probe according to any one of claims 1 to 11 , wherein the adhesive is an epoxy adhesive. A conductive backing block assembly for a two-dimensional ultrasonic transducer array comprising:
A plate of acoustic backing material;
And a circuit conductive traces, the plate and conductive circuit traces are arranged similarly to the conductive backing block assembly according to any one of claims 1 to 13, conductive backing block assembly.

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